This invention relates to a method of estimating the speed of a vehicle using wheel speed measurements including during incipient wheel lock conditions while the vehicle is being braked.
Wheel slip is a measure of the relative difference between vehicle speed and wheel speed. This parameter is used in antilock brake control systems and in traction control systems in determining when and whether to take corrective action in presenting excessive wheel slip. Since wheel slip is a measure of the relative difference between vehicle speed and wheel speed, any inaccuracies in the determination of the vehicle speed will result in inaccuracies in the calculated wheel slip which in turn may degrade overall system performance. Accordingly, it is desirable that the antilock brake system controller have an accurate indication of the vehicle speed.
Various methods have been proposed to determine vehicle speed. Some of these proposed methods involve the use of auxiliary devices such as vehicle body accelerometers, sonar vehicle speed sensors or radar speed sensors. However, these auxiliary devices add substantially to system complexity and cost.
Other proposed methods avoid this complexity and cost by estimating vehicle speed solely on wheel speed information and predetermined vehicle deceleration limits. In some of these methods, the vehicle speed follows wheel speed until an incipient wheel lockup condition is detected after which the vehicle speed is estimated based on a constant speed gradient represented by a predetermined vehicle deceleration limit. However, inaccuracies may be introduced by this method as a result of variations in the actual vehicle deceleration limit as a function of the road surface condition. For example, the deceleration attainable by braking a vehicle on a dry road bed may be substantially greater than the maximum possible vehicle deceleration obtainable on other surfaces such as gravel, snow, or a wet road bed. In addition, the wheel slip when an incipient wheel lockup condition occurs during braking may already be significant such that the wheel speed is not representative of the actual vehicle speed at the time of occurrence of the incipient wheel lockup condition.